Open Access
Issue
A&A
Volume 686, June 2024
Article Number A172
Number of page(s) 21
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202348174
Published online 11 June 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1. Introduction

The distinctive class of extragalactic maser sources known as hydroxyl (OH) megamasers (OHMs) emit non-thermal radiation from OH molecules, with two main lines at 1665/1667 MHz and two satellite lines at 1612/1720 MHz (McBride et al. 2013). This OH maser emission is believed to be stimulated by infrared (IR) radiation from their surroundings, with the OH line luminosity being closely tied to the far-infrared (FIR) luminosity (Baan 1985; Lockett & Elitzur 2008). Consequently, OHMs have typically been identified through targeted observations of luminous IR galaxies (LIRGs, LFIR >  1011L) or ultraluminous IR galaxies (ULIRGs, LFIR >  1012L) (see Baan et al. 1985; Darling & Giovanelli 2000, 2002a; Willett et al. 2012, 2011; Zhang et al. 2014).

High-sensitivity H I surveys, such as the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey (Haynes et al. 2018), as well as surveys conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) (Zhang et al. 2019) and with the Square Kilometer Array (SKA) and its precursors (see Roberts et al. 2021, and reference therein), have emerged as valuable tools for blind surveys aimed at detecting new OHMs. In these untargeted emission line surveys searching for neutral hydrogen, the redshifted OH lines at zOH can mimic an H I line with a redshift zH I if , where νHI = 1420.4 MHz and νOH = 1667.4 MHz (Roberts et al. 2021). Therefore, OHMs discovered through these blind surveys offer opportunities to validate the IR luminosity selection criteria employed in prior targeted surveys and to explore new environments conducive to OHM production (Suess et al. 2016).

The Arecibo ALFALFA survey represents the first endeavor to detect a sample of OHMs through a blind survey. This survey has reported nine confirmed OH megamasers and ten OHM candidates with no optical redshift but a potential redshift range of 0.16–0.22 (see Haynes et al. 2018). Optical spectroscopy is a primary method for confirming new OHMs. However, distinguishing between H I and OH in galaxies lacking spectroscopic redshifts continues to pose challenges. The studies of Roberts et al. (2021) and Suess et al. (2016) have demonstrated that the wide-field infrared survey Explorer (WISE) magnitudes and colors can serve as a means of distinguishing among H I and OHM host populations in the absence of spectroscopic redshifts. Moreover, Wu et al. (2023) conducted investigations into the arcsecond-scale radio continuum and OH line emission of a sample of known OHM galaxies with z >  0.15 and found that OH line emissions in known OHM galaxies exhibit compactness in arcsecond-scale Very Large Array (VLA) observations, often coinciding with radio continuum emission. These findings are consistent with the characteristics of low-redshift OHM galaxies documented in the literature (see Lo 2005, and references therein). In contrast, neutral hydrogen gases in galaxies are distributed on a wide range of scales, from sub-kpc to hundreds of kpc (Mundell & Shone 1999). Therefore, the detection of H I emission in nearby galaxies depends on brightness sensitivity in current radio interferometries, even with long integrations using the VLA in 5 arcsec resolution (Pedlar 2005). As a result, the distinctive properties of OH and H I line emission at arcsecond-scale observations support one hypothesis, namely, that arcsecond-scale spectral line emission observations can be utilized to identify new OHMs discovered in blind H I surveys.

This study presents the results of observations conducted with the Giant Meterwave Radio Telescope (GMRT) involving ten OHM candidates from the ALFALFA survey (Haynes et al. 2018). The primary objective of this research is to explore the characteristics of radio continuum and OH line emission on the arcsecond scale for these galaxies. Through this analysis, we aim to identify potential differences compared to known OHM galaxies and potentially confirm new OHM candidates. This endeavor also aims to validate the IR luminosity selection criteria used in previous targeted surveys and potentially uncover new environments conducive to OHM production (Suess et al. 2016). We provide details on our sample selection, observations, and data reduction in Sect. 2. The results, discussions, and conclusions are presented in Sects. 35. We have adopted a Hubble constant of H0 = 70 km s−1 Mpc−1, with Ωm = 0.3 and ΩΛ = 0.7, for calculating the luminosity distance of the targets in this study.

2. Observations and data reduction

2.1. GMRT observations

We conducted a total of 15 h of GMRT observations involving ten OHM candidates selected from the ALFALFA survey (see Table 1). We observed standard flux density calibrators (3C48, 3C147, and 3C286) every hour to correct for variations in amplitude and bandpass. These observational scans (each lasting about 6 min) are used for flux density, delay, and bandpass calibrations. Each target source was observed for about 50–70 min and a compact radio source was utilized as a phase calibrator, which was typically scanned for six minutes before and after observations of each target source (see Table 1). The images were created with a resolution of approximately ∼2–3 arcsec using the full GMRT array and 30–40 arcsec with the compact central array (see Table B.1). No self-calibration was performed due to the low signal-to-noise ratio (S/N) of both radio continuum and line emission images.

Table 1.

Observation parameters.

These observations were divided into three sessions carried out on different dates, specifically on January 17 and 20 and February 6, 2023. For this observation, we utilized band 5 of the GMRT, providing a total bandwidth of 400 MHz, spanning from 1060 to 1460 MHz. This entire bandwidth was further divided into 16 384 channels, which resulted in a frequency resolution of 24.414 kHz (6 km s−1). The selected frequency band allowed us to investigate a wide range of aspects, including broadband radio continuum emission and the redshifted OH lines at 1667 and 1665 MHz, as well as potential HI lines. However, at approximately 234 MHz of the spectra, particularly within the range from 1060 to 1294 MHz, there was substantial radio frequency interference (RFI) and we were unable to study the potiential H I emission of these galaxies. Consequently, for this study, we focused solely on the remaining 166 MHz band, spanning between 1294 and 1460 MHz, which was relatively free from RFI interference, enabling us to analyze the radio continuum and potential OH lines effectively.

2.2. Arcsecond-scale H I 21-cm line emission of nearby galaxies

Generally, OH masing regions are believed to be associated with star formation and the occurrence of OH megamaser activity is also correlated with IR colors (Pérez-Torres et al. 2021). Thus, OHM candidates linked to IR emissions are likely to demonstrate a high probability of OH line emission. At the same time, the detected line emission associated with IR emission could also originate from H I emission in nearby star-forming galaxies or (U)LIRGs. Therefore, it is crucial to compare the arcsecond-scale properties of H I emission in nearby star-forming galaxies and (U)LIRGs to distinguish OH megamasers from H I line emission.

In a study conducted by Liu et al. (2015), a set of star-forming galaxies featuring accessible interferometric observations of H I emission was assembled. Utilizing this sample, we gathered the integrated line flux density and H I mass from single-dish observations in the literature (see Table B.2). Additionally, we computed the H I mass within the central 2 kpc of the nuclei, relying on the provided H I surface densities of these galaxies by Liu et al. (2015). However, there is a shortage of available H I emission data for nearby (U)LIRGs due to the small atomic gas fraction, complex morphologies, and frequent observation of strong HI absorption features (see Liu et al. 2015). To address this gap, we compiled VLA-A/B and GMRT data for the (U)LIRGs in the Great Observatories All-sky LIRG Survey (GOALS) sample1. The GOALS sample, comprising 202 (U)LIRGs at z <  0.088, is a well-defined collection for enhanced IR emission in nearby galaxies (Armus et al. 2009). Approximately 40 sources in the GOALS sample were identified, with archived GMRT or VLA A/B data and exhibiting similar resolution to our current observations. Subsequently, we searched for single-dish H I observations of these 40 (U)LIRGs in the literature. Twelve sources were found to have H I absorption spectra from single-dish observations. These 12 sources were then excluded and the remaining 28 sources (listed in Table B.3) were selected for the study of arcsecond-scale H I emission. The H I mass and integrated line flux of galaxies in this work have been derived from each other using the available parameters, applying the equation from Giovanelli & Haynes (2015): MH I/M = 2.356 × 105SH ID2/(1+z), where SH I is the integrated flux in Jy km s−1 and D is the luminosity distance in Mpc, z is the redshift.

2.3. Data reduction

The GMRT and VLA archive data in this article were calibrated using the Common Astronomy Software Applications package (CASA2). The initial calibration of the GMRT data reduction process followed an online tutorial3. The data reduction processes for GMRT observations and VLA archive data follow similar procedures. Initially, we examined the data and executed flagging using different modes, such as “quack,” “TFCROP,” and “manual” modes. Subsequently, we conducted delay and bandpass calibration utilizing the flux calibrators. Next, we calibrated the phase and amplitude of the standard flux calibrators (3C48, 3C147, and 3C286) and the phase calibrator, transferring the flux density scale from the flux calibrator to the phase calibrator. Following self-calibration of the flux and phase calibrators, we performed additional manual flagging of the bad data and then recalibrated the data. Finally, we applied the final calibration tables to both the calibrators and target sources.

Subsequently, all calibrated data were imported into the DIFMAP package (Shepherd et al. 1997) to generate radio continuum and spectral line channel images. The radio continuum images were made using the RFI and line free channels. For the spectral line images, we subtracted the continuum emission based on a CLEANed map of the radio continuum emission. And since the high-resolution GMRT observations had the potential to resolve spectral line emissions, we also conducted calibration using data from the central GMRT array4 (GMRT-central), which consisted of fourteen antennas covering an area of approximately 1 km. The fundamental parameters for both data sets are outlined in Table B.1.

3. Results

3.1. Radio continuum emission from GMRT observations

We successfully detected radio continuum emission in four out of the ten OHM candidate sources (see Table 1). In Fig. 1, we present arcsecond-scale radio continuum emission images of these sources. It is evident that the radio continuum emission appears to be compact in three of these sources, with the fitted image components (see Table 1) being in close agreement with the beam full width at half maximum (FWHM). More details are provided in Table B.1. Notably, one source, AGC 219835, exhibits a double-elongated structure.

thumbnail Fig. 1.

OH line and radio continuum emission overlaid on SDSS R-band grey image. Green: circular region for generating the emission of the OH line from GMRT-central data, beam size of these sources, and parameters from Table B.1. Magenta: contour of OH line emission. The velocities range of the OH line emission: AGC 115713: 50 884.8–51 155.4 km s−1 (see Fig. 2); contour levels: 3σ (0.001 Jy beam−1) × (1 2 4 ...). For AGC 249507: 53 336.1–53 665.1 km s−1, contour levels: 3σ (0.000592 Jy beam−1) × (1 2 4 ...). Blue: L-band radio continuum emission and parameters from Table 1.

thumbnail Fig. 1.

continued.

Furthermore, we examined radio continuum images of our designated targets using three available VLA surveys: VLA Faint Images of the Radio Sky at Twenty centimeters (FIRST, Helfand et al. 2015), NRAO VLA Sky Survey (NVSS, Condon et al. 1998), and the Very Large Array Sky Survey (VLASS, Lacy et al. 2020). We retrieved the radio images of our sources from these surveys through the Canadian Initiative for Radio Astronomy Data Analysis (CIRADA) Image Cutout Web Service5. Subsequently, we examined and analyzed these images using CASA software, determining that the noise levels were approximately 0.31–0.71 mJy for NVSS images (1.4 GHz, resolution ∼45″), 0.09–0.19 mJy for VLA FIRST (1.4 GHz, resolution ∼5″), and 0.07–0.09 mJy for VLASS images (3 GHz, resolution ∼2.5″). In conclusion, we observe a consistent alignment between the radio continuum images from these surveys and our high-sensitivity observations (20–30 μJy beam−1) of these OHM candidates. This supports the detection of radio continuum emission in four of the OHM candidates.

3.2. OH lines emission from GMRT observations

We achieved a significant detection of OH line emission from two of the ten OHM candidates using GMRT-full data: AGC 115713 and AGC 249507 (see Figs. 1 and 2). The S/N values for both detections surpass 10 in both the image plane and the extracted line profiles with a frequency width of approximately 24 kHz (approximately 6 km s−1), with more details in Table 2. Additionally, we observed that the OH line emission in both sources are confined to single compact structures, with the size of the fitted components (see Table 2) being smaller than the beam size of our GMRT observations (see Table B.1). The OH line profiles exhibit multiple peaks, probably corresponding to the prominent 1667 MHz line. However, it is difficult to identify the 1665 MHz line feature because of the blending of the two lines, which is commonly shown in known OHM galaxies (Darling & Giovanelli 2000). Although the OH 1612 MHz line falls within our frequency range, we found that its signal remains below the 3σ threshold (approximately 3 mJy beam−1) in our observations of these sources.

Table 2.

OHM detected in our GMRT observations.

thumbnail Fig. 2.

OH line profiles of AGC 115713 (left) and AGC 249507 (right). For the spectra, the 1667.359 MHz line was used as the rest frequency for the velocity scale. Arrows indicate the expected velocity of the 1667.359 (left) and 1665.4018 (right) MHz lines, respectively. The arrow of the dotted line represents the expected velocity based on the redshift from OH line emission by Haynes et al. (2018). The arrow of the solid line is calculated by optical redshift from the LAMOST optical telescope survey. The OH line profiles of two sources were generated from a region about 7′′ × 7′′ and 5.65′′ × 5.26′′, respectively. The center coordinates and other parameters are listed in Table 2.

We centered at the peak frequency given by Haynes et al. (2018) and binned more channels to detect potential signals for the unconfirmed sources. In Figs. B.1B.3, we present the line profiles of unconfirmed sources, generated by binning the data every two channels at selected circular regions. In Figs. B.2 and B.3, the dirty map displays the binning of all line channels within a bandwidth of 5 MHz centered at the peak frequency. In the case of AGC 749309, we observed narrow line features that appear to be distributed in several regions around its potential host galaxy (see Appendix A.5 and Fig. B.2). However, for the other seven candidates, we did not see any visible line features (see Fig. B.3). Subsequently, we utilized the data from antennas in the low-resolution GMRT-central and further detected three potential sources of spectral line emission: AGC 102850, AGC 115018, and AGC 749309 (see Fig. B.1). These line profiles peak at a frequency similar to that observed in the ALFALFA survey by Haynes et al. (2018). However, these detections register at a noise level lower than 3σ in the image plane, preventing any accurate determination of the positions of these detections (see Appendix A and Fig. 1).

4. Discussion

4.1. Confirmation of two OHM galaxies from GMRT observations

The spatial correlation between OH masers and radio continuum emission has received substantial support from both physical models and arcsecond-scale radio observations (Pihlström 2005; Wu et al. 2023). Since both radio continuum and OH line emission in OHM galaxies are primarily linked to starbursts in the nuclei region, typically within 100 pc (see Pihlström 2005; Hess et al. 2021), they tend to be compact and coexist at arcsecond-scale radio observations. Consequently, it is widely accepted that OHM emission is unlikely to be resolved at arcsecond-scale observations, and OHM emission is generally associated with radio continuum emission at this scale (Wu et al. 2023).

Our observations indicate that the OH line emission and radio continuum emission of the two sources, AGC 115713 and AGC 249507, are distributed within regions smaller than the beam size of the GMRT (see Tables 1 and 2, and Sect. 3.2). This observation is well aligned with the characteristics typically found in OHM galaxies observed at arcsecond-scale radio observations.

Generally, optical spectroscopy is a crucial method for confirming new OHMs. We have made efforts to obtain redshift information from the literature and optical surveys, including Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey. We found that only AGC 249507 has an available redshift from the LAMOST survey6 and this redshift aligns with the system velocity derived from the OH line emission (see Fig. 2).

Furthermore, Suess et al. (2016) proposed a method to distinguish OH megamasers from H I line emitters without relying on optical spectroscopy. This method involves using the WISE IR colors and magnitudes, incorporating four IR criteria: [3.4]−[4.6]> 0.6, [4.6]−[12]> 3.0, [22]> 4.8, and [3.4]< 15.3, which can be depicted as three zones in Fig. 3. We collected the WISE color data for our sources from the NASA/IPAC INFRARED science archive webpage7. Using these criteria, we were only able to confirm AGC 115713 as an OHM galaxy (see Table 3 and Fig. 3). While AGC 249507 does not entirely satisfy these IR criteria (see Table 3 and Fig. 3), its confirmation through optical redshift information from the LAMOST survey demonstrates the effectiveness of our GMRT spectral line observations in confirming new OH megamasers in this sample.

Table 3.

IR properties of OHM candidates.

Similarly, we conducted tests on the H I galaxies (listed in Tables B.2 and B.3) using the WISE four IR criteria. Our findings indicate that these criteria have excluded all of these H I galaxies (see Fig. 3). This outcome aligns with the results presented by Suess et al. (2016), demonstrating that these IR cuts can successfully eliminate 99.3% of nearby H I galaxies. Therefore, the only OHM candidate (AGC 115713) passing the IR cuts is more likely to be an OHM rather than a nearby H I galaxy. Additionally, the same IR cuts were applied to the known OHM galaxies documented in Zhang et al. (2014). The results show that these IR criteria have also excluded all 51 known OHM galaxies at z <  0.1. Among the known OHM galaxies at z >  0.1, approximately 54% of these sources successfully pass the IR cuts. Consequently, the IR cuts may not be as effective in confirming new OHM galaxies as they have been in excluding nearby galaxies. However, it is noteworthy that the known OHM galaxies at z >  0.1 are likely concentrated in regions close to the shaded areas shown in Fig. 3. Furthermore, the two confirmed OH candidates in this study are also distributed in a region consistent with known OHM galaxies at z >  0.1 (see Fig. 3).

thumbnail Fig. 3.

Ten OHM candidates in the WISE color–magnitude and color–color space from Suess et al. (2016). The red pentagrams are OHMs confirmed in this work (AGC 115713 and AGC 249507). The blue points represent the OHM candidates not confirmed in this work. The three zones (1–3) represent the IR cuts to separate H I and OH emitters (Suess et al. 2016).

4.2. IR properties of OHM candidates

Generally, OH megamasers (OHMs) are recognized as highly luminous masers, typically found in [U]LIRGs. Consequently, all previously documented OHM galaxies are invariably linked to high IR luminosities (LFIR >  1011L), as determined from the 60 μm and 100 μm IR flux densities provided by the IRAS survey (Darling & Giovanelli 2000; Roberts & Darling 2023). Notably, it is worth mentioning that only one of the ten OHM candidates, AGC 249507, is accompanied by available 60 μm and 100 μm flux data in the IRAS survey (see Table 3). Zhang et al. (2014) demonstrated that the MIR properties of OHMs and non-OHMs adhere to the power-law model rather than a single blackbody model. Therefore, for the remaining nine OHM candidates, we applied a power-law fit (F ∝ να) to the data from the four WISE bands for each source and subsequently extrapolated the FIR flux values at 60 μm and 100 μm, respectively. Subsequently, we calculated the FIR flux density of these sources using the equation from Baan & Klöckner (2006): SFIR = 1.26 × 10−14 (2.58F60 + F100) W m−2, where F60 and F100 is in units of Jy, then we calculated the FIR luminosity, LFIR, based on this flux density using equation , where DL is the luminosity distance. These results are presented in Table 3 .

Our analysis reveals a significant difference in IR luminosities between the two confirmed OHM candidates and those that remain unconfirmed (see Table 3). All ten galaxies have IR flux densities that fall below the selection criterion of f60 μm >  0.6 Jy, as originally defined by Darling & Giovanelli (2002b) for targeted observations. Notably, the IR luminosities of the two confirmed OHM candidates also fall within the category of LIRGs, which are typically the host galaxies of known OHM galaxies. In contrast, the unconfirmed OHM candidates exhibit IR luminosities approximately one magnitude lower than those observed in known OHM galaxies. Consequently, these unconfirmed OHM candidates cannot be categorized as (U)LIRGs, which are the characteristic host galaxies of known OHM galaxies. If these sources are ultimately validated through optical redshift measurements or the detection of a second radio spectral line, they might represent a new category of OHM galaxies that exist within different environmental conditions, distinct from (U)LIRGs.

4.3. Radio continuum emission association of the OH line

We have successfully detected radio continuum emission in four out of the ten OHM candidates, including two confirmed OHM galaxies. In addition, we have compiled the VLA NVSS or FIRST flux densities for 115 previously discovered OH maser galaxies, as reported by Zhang et al. (2014), and subsequently calculated the distributions of radio luminosity at various redshifts (see Fig. 4). Our analysis revealed that the upper limit of radio luminosity for six sources without detectable radio continuum emission deviates significantly from the known OHM galaxies. Concurrently, we found that two confirmed OHM sources, AGC 115713 and AGC 249507, align well with the distribution exhibited by known OHM galaxies (see Fig. 4). Glowacki et al. (2022) reported the first untargeted detection of an OHM at z >  0.5, which is also the highest redshift detection of such a system to date. The radio continuum emission is about 0.42 mJy at L-band, and the corresponding luminosity is about 23.6 W Hz−1, which is also consistent with the OHM galaxies at low-redshift as shown in Fig. 4.

thumbnail Fig. 4.

L-band radio continuum luminosity distributions of OHM galaxies along the redshift. The red stars and blue points stand for the confirmed and unconfirmed OHM candidates in this work.

From Fig. 4, it is evident that known OHM galaxies with high radio luminosities (Lradio) are relatively rare, with only three sources (IRAS 01569−2939, IRAS 02483+4302, and IRAS 13451+232) falling in the range of 24.8–26.3 W Hz−1. This finding supports the prevailing idea that radio continuum emission in OHM galaxies primarily arises from star formation processes (Pérez-Torres et al. 2021). We further investigated whether these galaxies represent genuine detections and found that, out of the three OHM galaxies with high Lradio, only IRAS 01569−2939 has been definitively confirmed as a detection, using the Green Bank Telescope (GBT) telescope (Willett et al. 2012). The other two sources, IRAS 02483+4302 and IRAS 13451+232, remain tentative, as they were only reported as potential detections by Kazes et al. (1989) and Dickey et al. (1990), respectively, and may require further confirmation (Darling & Giovanelli 2002a). Therefore, while sources with high radio luminosities may still be outliers, this consideration also applies to AGC 219835 in our sample, which also exhibits high radio luminosities (approximately 25.3 W Hz−1, see Fig. 4).

4.4. Comparison of arcsecond-scale properties of H I and OH line emission

Typically, OH megamaser emission is thought to be confined within a 100 pc region of the nucleus (see Hess et al. 2021, and references therein). Consequently, the physical scale of OH megamaser emission results in distribution within a region with angular sizes smaller than 1″, even for low-redshift OHM host galaxies (Lo 2005). Wu et al. (2023) found that the OH line profiles of OHM galaxies at z >  0.15, measured from VLA-A observations, exhibit no significant differences compared to those from single-dish Arecibo observations, except for variations in some narrow OH line features caused by interstellar scintillation due to their small sizes. Given that OH candidates from the ALFALFA blind emission line survey generally have a redshift of zOH ∼ 0.2 (Suess et al. 2016), these candidates might share similar properties with known OHM galaxies studied in Wu et al. (2023). Specifically, the OH line emission is confined within a region smaller than 1″, and the line profiles are consistent with those from single-dish observations.

Unlike OHM emission, H I gas in nearby galaxies is believed to be distributed on scales ranging from sub-kpc to hundreds of kpc, mostly extended to a scale larger than 2 kpc (Wang et al. 2016). Consequently, arcsecond-scale observations of H I emission may primarily capture only the mass distributed in the nuclear region of these galaxies, resulting in significant differences from H I emission obtained from single-dish observations. We derived the ratio of integrated H I flux from the 2 kpc scale and from single-dish observations of nearby star-forming galaxies and (U)LIRGs (see Sect. 2.2). It is evident that nearly all of these galaxies exhibit H I emission from the 2 kpc region, accounting for less than 10% of the total H I emission (see Fig. 5 and Table B.2). We obtained the spectra of the (U)LIRGs selected for this work (see Sect. 2.2 either from literature or based on the archive data reduced in this work; see Table B.3). The results show that most nearby (U)LIRGs (19/28) display H I absorption spectra at our selected interfermotric observations, with only six sources exhibiting diffused H I emission. Meanwhile, the H I line profiles extracted from the central 10′′ × 10′′ region (1.9–5 kpc; see Fig. B.4) of these (U)LIRGs are also significantly different from those from single-dish observations, and the integrated line flux constitutes less than 10% of the total emission H I (see Figs. 5, B.4, and B.5).

thumbnail Fig. 5.

H I integrated line flux ratio (FI/FS) of nearby starforming galaxies and (U)LIRGs from Tables B.3 and B.2. FS and FI represent the H I integrated line flux from the single-dish and interferometric array, respectively.

The ALFALFA survey is a blind emission line survey for H I galaxies at z ≤ 0.06 (Haynes et al. 2018). The OH candidates identified by Haynes et al. (2018) would actually have a redshift z <  0.024 if they were H I emission rather than OH emission based on their peak line frequency. It should be noted that the two confirmed OHM galaxies (AGC 115713 and AGC 249507) exhibit several line components (see Fig. 2); thus, a direct comparison with the line profiles from ALFALFA is not possible, as they are not available in the literature. The brightest feature has an integrated flux density of about 1.66 and 1.35 Jy km s−1 (see Table 2), while the integrated flux density from the ALFALFA survey is about 1.36 and 1.65 Jy km s−1, respectively. The differences are approximately 15% for the two sources. However, our line profiles also reveal weak line features, and the total integrated flux density from our observations for the two sources is about 1.97 Jy km s−1 and 2.29 Jy km s−1 (see Appendix A for notes on each source), respectively. Therefore, the integrated flux density from the ALFALFA survey is approximately 70% of the flux detected in our work.

This discrepancy could be attributed to our observations, which were carried out at nearly twice the sensitivity of the ALFALFA survey. Then, the velocity range may be different, as the weak line features of the two sources are lower than the noise level (∼2 mJy) of the ALFALFA survey (see Fig. 2 and Haynes et al. 2018), along with uncertainties in the integrated flux density from both our observation and the ALFALFA survey. This result suggests that the line flux density of the two sources is not resolved with our arcsecond-scale observations, even for the weak line features. This is consistent with the properties of known OHM galaxies at similar redshifts, as shown in Wu et al. (2023).

For the eight unconfirmed OHM candidates, the line emission is not detected with the GMRT-full data, indicating that the spectral line emission initially detected by the ALFALFA survey is possibly resolved. However, we estimate that the upper limit of the integrated flux with Eq. (3) (RMS × FWHM) will not be ten times lower than the integrated flux given by Haynes et al. (2018), which is the typical ratio for H I emission of nearby galaxies (see Fig. 5). This is due to the low S/N of these sources (S/N of ∼5 Haynes et al. 2018). Consequently, higher-sensitivity observations might be useful to further confirm whether these sources are significantly resolved at the central region of these galaxies, similar to nearby H I galaxies shown in Fig. 5.

4.5. Implications for confirming new OHMs from the blind survey

Using GMRT observations, we have successfully confirmed two of the ten OHM candidates initially identified in the ALFALFA survey by Haynes et al. (2018). Our confirmation is based on arcsecond-scale radio continuum and OH line emission, as well as the characteristics exhibited by known OHM galaxies. Thus, the spectral line emission observed in the unconfirmed OHM candidates by the ALFALFA survey may not necessarily be OH line emissions. They could potentially be H I emissions in clusters or even some other unidentified type of radio line emission, as suggested in Giovanelli & Haynes (2015). Therefore, conducting arcsecond-scale radio continuum and spectral line observations proves to be an effective approach for discovering new OHM galaxies that share common features with known OHM galaxies.

From the currently available VLA survey images (NVSS, FIRST, and VLASS), we have found that all OHM galaxies listed in Zhang et al. (2014) have been linked to radio continuum detections when they are located within the sky coverage of these surveys. This finding supports the idea that radio continuum emission plays a role in generating OH line emission (Sotnikova et al. 2022a,b). Similarly, we can further narrow down these results to only AGC 115713, 249507, 749309, and 219835 remaining as potential OHM candidates. This determination is based on their radio continuum detections in VLA surveys and the alignment of their radio luminosity with that of known OHM galaxies. This initial selection process using radio continuum emission enables us to reduce our sample size by 60% before conducting high-resolution spectral line observations. Hence, available radio continuum emission can be used as an initial screening criterion for OHM candidates, in agreement with the view of Koribalski et al. (2020).

Among the ten OHM candidates, only the two LIRGs with the highest IR luminosity in this sample have been confirmed to be OHM galaxies (see Table 3). Suess et al. (2016) suggested that there is no previously unknown OHM producing population at z ∼ 0.2 and confirmed the validity of the IR selection method used in previous targeted surveys for OHM galaxies. These findings imply that IR luminosity could potentially serve as a preliminary selection criterion, akin to the previous targeted surveys, based on known OHM galaxies for potential OHM candidates. Although the available far-IR flux density and high far-IR luminosities cannot directly confirm the OHM galaxies found in blind H I surveys (Darling 2007), they are aptly correlated with the OH luminosity (Darling & Giovanelli 2002a) and play a crucial role in OHM formation (Lockett & Elitzur 2008). In cases where other H I blind surveys identify OHM candidates without optical redshift information, the absence of associated IR flux density detection or significantly lower calculated IR luminosity compared to known OHM galaxies (e.g., similar to eight out of ten of our OHM candidates with much lower IR luminosities than known OHM galaxies, as shown in Table 3) may indicate a higher likelihood that they are not the OHM detections. Then, the number of OHM candidates need for further confimation might be significantly reduced.

Optical spectroscopic redshifts remain the most reliable method for confirming new OHM galaxies. For unconfirmed galaxies, further spectral observations, either optical or radio, may be necessary to reach a definitive conclusion. Additionally, it is important to note that our current assessment of unconfirmed OHM galaxies in this study is based on existing knowledge about known OHM galaxies. We acknowledge that arcsecond-scale spectral line observations of known OHM galaxies, available in the literature, are still limited, particularly when dealing with diffused OHM emission. The redshift range of known OHM galaxies spans from z = 0.01 to z = 0.277 (Darling & Giovanelli 2002a; Zhang et al. 2024), with the addition of two recently discovered OHM galaxies at z = 0.523 and z = 0.709 by Glowacki et al. (2022) and Jarvis et al. (2023), respectively. Considering the redshift range of known OHM galaxies, arcsecond-scale observations may potentially reveal extended emissions, which could be attributed to intrinsic diffusion or the presence of multiple masing nuclei, as suggested by Darling & Giovanelli (2000). These emissions should correspond to a physical scale ranging from several hundred parsecs to tens of kiloparsecs. However, high-sensitivity observations of known OHM galaxies are limited to less than two dozen in the literature. It appears that no diffuse OH maser emission has been detected in these observations, supporting the idea of the compact nature of OHM emission (Wu et al. 2023).

As the physical scale of H I gas in nearby galaxies is predominantly extended to sizes larger than 2 kpc (Wang et al. 2016), we adopted a 2 kpc physical scale for distinguishing between the H I host and potential OHM candidates. This scale corresponds to about 1″ at zH I = 0.1 and a potential OHM candidate at zOH = 0.29. Given that the H I surveys from ALFALFA are focused on H I galaxies with z <  0.06, arcsecond-scale observations are suitable for this study. However, Roberts et al. (2021) demonstrated that the upcoming H I surveys will mostly extend to redshifts zH I much higher than 0.1. In these galaxies, the 2 kpc scale will be smaller than 1″, potentially necessitating sub-arcsecond and even mas-scale observations to distinguish H I emission from OH emission. Furthermore, Roberts et al. (2021) indicated that the next generation H I surveys will detect an unprecedented number of OHM galaxies, most of which will lack spectroscopic redshifts. Therefore, expanding high-sensitivity and high-resolution observations to a larger number of known OHM galaxies can serve to either confirm the compact nature of OHM emission or detect OHM maser emission with diffused characteristics, similar to the unconfirmed candidates in this study. These results can contribute to a deeper understanding of OH line emission properties and further confirming of OHM candidate from other H I surveys.

5. Summary

We present the results of the GMRT observations of 10 OHM candidates identified in the ALFALFA survey. Among these candidates, two sources (AGC 115713 and AGC 249507) exhibit compact OHM line emission and are associated with radio continuum emission, consistent with the typical characteristics of known OHM galaxies. Additionally, these two confirmed OHM galaxies display substantial FIR luminosity, aligning with the general model of OHM galaxies. Furthermore, AGC 249507 has an optical redshift recorded in the LAMOST survey and AGC 115713 meets the IR selection criteria established in the literature, which also supports our confirmation of the two sources as OHM host galaxies rather than nearby H I galaxies.

No significant spectral line emission was detected in the remaining 8 OHM candidates using our GMRT-full dataset. Furthermore, six of these candidates did not exhibit radio continuum emission, even with our high-sensitivity observations (approximately 30 μJy beam−1). Upon analysis of the GMRT-central data, we found that three out of the eight candidates potentially exhibit spectral line emission, with central frequencies roughly consistent with those reported in the ALFALFA survey. Meanwhile, the absence of radio continuum emission and the presence of diffuse spectral line emission distinguish the unconfirmed OHM candidates from known OHM galaxies documented in the literature.

These findings indicate that arcsecond-scale observations are an effective means of confirming OHM galaxy candidates. The spectral line emission observed in unconfirmed sources by the ALFALFA survey is more likely to be associated with H I emission or other unidentified spectral lines (Giovanelli & Haynes 2015). To further validate the nature of the radio line emission observed in the ALFALFA survey, additional measures such as optical spectroscopic redshift or line emission at other wavelength bands are necessary. These measures will help determine whether these unconfirmed candidates represent a new category of OHM galaxies displaying diffuse OH line emission resolved on arcsecond-scale observations.


Acknowledgments

We thank the anonymous referee for the constructive comments and suggestions which substantially improved this paper. This work is supported by NSFC grants (Grant No. U1931203,12363001). The Giant Meterwave Radio telescope (GMRT) is the most sensitive synthetic aperture Radio telescope in the meter band. It is operated by a department of the Tata Institute of Basic Research – the National Radio Astronomical Center (NCRA) in the United States. Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.

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Appendix A: Notes on individual sources

A.1. AGC102850

From the data obtained from GMRT-central, we have identified potential spectral line emission with a central frequency of approximately 1421.5 MHz, which is slightly lower than the results obtained from the ALFALFA survey (see Fig. B.1). It is worth noting that this spectral line emission appears to be diffuse, hindering our ability to pinpoint its optical counterpart (see Fig. 1). Furthermore, we have not detected any radio continuum emission in this galaxy, with a noise level at approximately 0.032 mJy/beam.

A.2. AGC115018

Utilizing the data acquired from GMRT-central, we have successfully detected potential spectral line profiles, as illustrated in Fig. B.1. Specifically, the OH line emission was extracted from the vicinity of the optical counterpart as provided by Haynes et al. (2018; see Fig. 1). The central frequency aligns with the findings of the ALFALFA survey (see Fig. B.1). We have not detected any radio continuum emission, with a noise level approximately at 0.026 mJy/beam.

A.3. AGC115713

The SDSS image of this source reveals the presence of two nuclei located in the southeast (SE) and northwest (NW) regions. Our GMRT observations have confirmed that the OH line emission and radio continuum emission spatially coincide with the NW nucleus, as illustrated in Figure 1. This confirms the optical counterpart identification of SDSS 014135.2+165731, a selection made by Haynes et al. (2018). The OH line profile displays two distinct peaks corresponding to the OH 1667 MHz line. The two line features are about 1.66 Jy km s−1 and 0.31 Jy km s−1, respectively. We have obtained a radio continuum flux of 1.23 mJy at 3 GHz from the VLASS survey and a flux of 1.73 mJy at 1.4 GHz observed by GMRT, resulting in a spectral index of -0.45.

A.4. AGC249507

This source has an available optical redshift of z=0.178139, obtained from LAMOST survey data release 8. Importantly, this redshift matches the OH line frequency, as shown in Figure 2. Furthermore, both the radio continuum emission and OH line emission from this source exhibit spatial correlation, with the fitted component size being smaller than the beam size used in our GMRT observations (see Tables 1 and 2). The OH line profile reveals the presence of multiple components with a resolution of approximately 6 km/s. The integrated line fluxes of the three features (see Fig. 2), ranging from low to high velocities, are approximately 0.65, 1.35, and 0.29 Jy km s−1. Additionally, our analysis indicates that the radio continuum flux for this source is approximately 2.4 mJy at 3 GHz from VLASS and 3.0 mJy at 1.4 GHz from our GMRT observations, resulting in a spectral index of -0.28.

A.5. AGC749309

We have identified two radio continuum components near the potential OH emission source initially identified in the ALFALFA survey by Haynes et al. (2018). The integrated flux of these two components is 2.91 mJy and 1.2 mJy, respectively, with one situated in the northwest (NW) and the other in the southeast (SE). This is illustrated in Fig. 1 and detailed in Table 1. However, we have not detected significant spectral line emissions associated with these two radio continuum components. Instead, the spectral line emission appears to be distributed in the region between these two radio components (designated as D1-3, as shown in Fig. B.2), as well as in the vicinity of SDSS 101101.1+274012 (referred to as D2, see Fig. B.2), which is the optical counterpart identified by Haynes et al. (2018). Due to the diffuse distribution of the radio spectral line emission and the absence of any prominent components exceeding 3 σ in the image plane, it is evident that higher sensitivity observations will be necessary to accurately pinpoint the precise location of the spectral line emission.

Haynes et al. (2011) initially classified the radio spectral line as H I emission, while Haynes et al. (2018) reclassified it as a potential OHM candidate. Despite our successful re-detection of the spectral line emission associated with this source, the radio spectral line emission is neither compact nor linked to radio continuum emission. Moreover, it lacks a clear association with prominent IR emission, as indicated by available data from IRAS and WISE surveys. Consequently, these characteristics suggest that this source does not exhibit the typical properties associated with OH megamaser emission.

A.6. AGC219835

The radio continuum emission exhibits an elongated structure, spanning approximately 30", which likely exceeds the scale of the galaxy, as evident from the SDSS image (see Fig. 1). The optical counterpart associated with the detected radio continuum emission is likely the quasar 4C+32.38. The radio continuum flux measures approximately 144.7 mJy at 3 GHz according to VLASS data, while GMRT observations report a flux of 364 mJy at 1.4 GHz. Our analysis of the WISE data and the IRAS survey did not yield any 3 σ detections. These characteristics do not align with the typical properties of an OHM galaxy, further strengthening our conclusion that this OHM candidate is not confirmed.

Appendix B: Additional tables and figures

Table B.1.

Parameters of the OHM candidates from ALFALFA.

Table B.2.

H I host galaxies from some nearby star-forming galaxies.

Table B.3.

H I host galaxies from GOALs sample.

thumbnail Fig. B.1.

OH line profiles generated using the data from the low-resolution GMRT-central. The OH line profiles were extracted from a circular region with sizes of 60"×60" centered at the position given by Haynes et al. (2018; see Table B.1 and Fig. 1). The arrow represents the peak frequency of the line profiles given by Haynes et al. (2018). The red line profiles of AGC 749309 represent the ALFALFA spectra of this source from Haynes et al. (2011).

thumbnail Fig. B.2.

Radio line profiles of AGC 749309 generated using the high-resolution GMRT-full. Top left: the R-band SDSS image of AGC 749309. The green circles stand for the regions where we extract the spectral line emission. The size of the large and smaller cycles are 20"×20" and 5"×5", respectively. The extracted line profiles from these regions are present in other panels of this figure.

thumbnail Fig. B.3.

OH line profiles generated using the high-resolution GMRT-full. Left: The dirty map of possible OH line emission about 5 MHz bands centered at the peak frequency given by Haynes et al. (2018). The OH line profiles are mostly generated from green circles with size of about 20"×20" except for AGC219835, which is 40"×40".

2

thumbnail Fig. B.3.

continued

2

thumbnail Fig. B.3.

continued.

thumbnail Fig. B.4.

H I line and radio continuum emission overlaid on an optical R-band grey image (PanSTARRS). The magenta contours represent the integrated H I line emission in selected velocity ranges, as indicated in Fig. B.5. IRAS F00163-1039: 7998.06-8359.56 km s−1, IRAS F02152+1418: 3600-4200 km s−1, IRAS F13373+0105: 6425.81-6995.61 km s−1, IRAS F15276+1309: 3697.75-4277.18 km s−1, IRAS F20221-2458: 3011.4-3152.88 km s−1, IRAS F23488+1949: 4180.91-4626.35 km s−1. The initial contour levels for these sources are at the 2 σ level, with values of 0.368, 0.298, 0.154, 0.339, 0.439, and 0.433 mJy/beam, respectively. Only IRAS F23448+1949 has a second contour (2σ ×2), while other sources have only the first contour. In blue, the emission of the L-band radio continuum is depicted, with the first contour levels set at level 3 σ -specifically, 1.062, 0.992, 0.672, 0.765, 1.431, and 2.121 mJy/beam, respectively. The contour levels follow a pattern (1 2 4 8,...). The ellipses at the bottom left of each panel indicate the beam size of the interferometric observations, as depicted in Table B.3.

thumbnail Fig. B.5.

H I line profiles of (U)LIRGs.The arrow in the plot indicates the systemic velocity. For the interferometric array, the H I line profiles are produced within a circular region of approximately 10"×10", centered around the nuclei center of these galaxies, as illustrated in Fig. B.4. Exceptionally, in the cases of IRAS F00163-1039 and IRAS F13373+0105, each possessing two nuclei, the line profiles are generated from the south and southeast nuclei of the two sources, respectively. The references for the single-dish line profiles can be found in Table B.3.

All Tables

Table 1.

Observation parameters.

Table 2.

OHM detected in our GMRT observations.

Table 3.

IR properties of OHM candidates.

Table B.1.

Parameters of the OHM candidates from ALFALFA.

Table B.2.

H I host galaxies from some nearby star-forming galaxies.

Table B.3.

H I host galaxies from GOALs sample.

All Figures

thumbnail Fig. 1.

OH line and radio continuum emission overlaid on SDSS R-band grey image. Green: circular region for generating the emission of the OH line from GMRT-central data, beam size of these sources, and parameters from Table B.1. Magenta: contour of OH line emission. The velocities range of the OH line emission: AGC 115713: 50 884.8–51 155.4 km s−1 (see Fig. 2); contour levels: 3σ (0.001 Jy beam−1) × (1 2 4 ...). For AGC 249507: 53 336.1–53 665.1 km s−1, contour levels: 3σ (0.000592 Jy beam−1) × (1 2 4 ...). Blue: L-band radio continuum emission and parameters from Table 1.

In the text
thumbnail Fig. 1.

continued.

In the text
thumbnail Fig. 2.

OH line profiles of AGC 115713 (left) and AGC 249507 (right). For the spectra, the 1667.359 MHz line was used as the rest frequency for the velocity scale. Arrows indicate the expected velocity of the 1667.359 (left) and 1665.4018 (right) MHz lines, respectively. The arrow of the dotted line represents the expected velocity based on the redshift from OH line emission by Haynes et al. (2018). The arrow of the solid line is calculated by optical redshift from the LAMOST optical telescope survey. The OH line profiles of two sources were generated from a region about 7′′ × 7′′ and 5.65′′ × 5.26′′, respectively. The center coordinates and other parameters are listed in Table 2.

In the text
thumbnail Fig. 3.

Ten OHM candidates in the WISE color–magnitude and color–color space from Suess et al. (2016). The red pentagrams are OHMs confirmed in this work (AGC 115713 and AGC 249507). The blue points represent the OHM candidates not confirmed in this work. The three zones (1–3) represent the IR cuts to separate H I and OH emitters (Suess et al. 2016).

In the text
thumbnail Fig. 4.

L-band radio continuum luminosity distributions of OHM galaxies along the redshift. The red stars and blue points stand for the confirmed and unconfirmed OHM candidates in this work.

In the text
thumbnail Fig. 5.

H I integrated line flux ratio (FI/FS) of nearby starforming galaxies and (U)LIRGs from Tables B.3 and B.2. FS and FI represent the H I integrated line flux from the single-dish and interferometric array, respectively.

In the text
thumbnail Fig. B.1.

OH line profiles generated using the data from the low-resolution GMRT-central. The OH line profiles were extracted from a circular region with sizes of 60"×60" centered at the position given by Haynes et al. (2018; see Table B.1 and Fig. 1). The arrow represents the peak frequency of the line profiles given by Haynes et al. (2018). The red line profiles of AGC 749309 represent the ALFALFA spectra of this source from Haynes et al. (2011).

In the text
thumbnail Fig. B.2.

Radio line profiles of AGC 749309 generated using the high-resolution GMRT-full. Top left: the R-band SDSS image of AGC 749309. The green circles stand for the regions where we extract the spectral line emission. The size of the large and smaller cycles are 20"×20" and 5"×5", respectively. The extracted line profiles from these regions are present in other panels of this figure.

In the text
thumbnail Fig. B.3.

OH line profiles generated using the high-resolution GMRT-full. Left: The dirty map of possible OH line emission about 5 MHz bands centered at the peak frequency given by Haynes et al. (2018). The OH line profiles are mostly generated from green circles with size of about 20"×20" except for AGC219835, which is 40"×40".

In the text
thumbnail Fig. B.4.

H I line and radio continuum emission overlaid on an optical R-band grey image (PanSTARRS). The magenta contours represent the integrated H I line emission in selected velocity ranges, as indicated in Fig. B.5. IRAS F00163-1039: 7998.06-8359.56 km s−1, IRAS F02152+1418: 3600-4200 km s−1, IRAS F13373+0105: 6425.81-6995.61 km s−1, IRAS F15276+1309: 3697.75-4277.18 km s−1, IRAS F20221-2458: 3011.4-3152.88 km s−1, IRAS F23488+1949: 4180.91-4626.35 km s−1. The initial contour levels for these sources are at the 2 σ level, with values of 0.368, 0.298, 0.154, 0.339, 0.439, and 0.433 mJy/beam, respectively. Only IRAS F23448+1949 has a second contour (2σ ×2), while other sources have only the first contour. In blue, the emission of the L-band radio continuum is depicted, with the first contour levels set at level 3 σ -specifically, 1.062, 0.992, 0.672, 0.765, 1.431, and 2.121 mJy/beam, respectively. The contour levels follow a pattern (1 2 4 8,...). The ellipses at the bottom left of each panel indicate the beam size of the interferometric observations, as depicted in Table B.3.

In the text
thumbnail Fig. B.5.

H I line profiles of (U)LIRGs.The arrow in the plot indicates the systemic velocity. For the interferometric array, the H I line profiles are produced within a circular region of approximately 10"×10", centered around the nuclei center of these galaxies, as illustrated in Fig. B.4. Exceptionally, in the cases of IRAS F00163-1039 and IRAS F13373+0105, each possessing two nuclei, the line profiles are generated from the south and southeast nuclei of the two sources, respectively. The references for the single-dish line profiles can be found in Table B.3.

In the text

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